1. Field of the Invention
The present invention relates to an apparatus for controlling the speed of an elevator, which controls an AC electric motor that drives the cage, by using a power rectifier (hereinafter referred to as a converter) comprising thyristors and a power inverter (hereinafter referred to as an inverter) comprising a transistor and a diode.
2. Description of the Prior Art
FIG. 1 is a block diagram illustrating the structure of a conventional apparatus for controlling the speed of an elevator, which is disclosed, for example, in Japanese Patent Laid-Open No. 162978/1981, wherein reference numeral 1 denotes a three-phase AC power source, reference numeral 2 denotes a vonverter which consists of thyristors and which converts an AC power source voltage into a direct current, numeral 3 denotes a capacitor for smoothing the output of the converter, numeral 4 denotes a converter which consists of a transistor and a diode, and which converts the DC voltage smoothed by the capacitor 3 into an alternating current of which the voltage and frequency can be changed, numeral 5 denotes a three-phase induction motor (hereinafter simply referred to as an electric motor) that is an AC electric motor, numeral 6 denotes a speed detector such as tachometer generator which is directly coupled to the electric motor 5, numeral 7 denotes a sheave rotated by the electric motor 5, numeral 8 denotes a rope wound on the sheave, numeral 9 denotes a cage coupled to an end of the rope, and numeral 10 denotes a counter weight coupled to the other end of the rope.
Reference numeral 11 denotes a speed instruction generator which generates a speed instruction signal 11a, numeral 12 denotes a speed control operation circuit which compares the speed instruction signal 11a with a speed signal 6a of the speed detector 6 to produce an output instruction signal 12a for the converter 2 and a slip frequency instruction signal 12b for the inverter 4, reference numeral 13 denotes a voltage detector which detects the voltage across the ends of the capacitor 3, numeral 14 denotes a voltage control operation circuit which compares the output instruction signal 12a with a voltage signal 13a of the voltage detector 13 to produce a voltage instruction signal, numeral 15 denotes a phase circuit which generates gate pulses to control the thyristors that constitute the converter 2 in response to the voltage instruction signal, numeral 16 denotes a voltage instruction generator circuit which generates a voltage instruction signal of a sinusoidal wave responsive to the speed signal 6a and the slip frequency instruction signal 12b, numeral 17 denotes a voltage detector circuit which detects the output voltage of the inverter 4, numeral 18 denotes a pulse-width modulation comparator which compares the voltage instruction signal of the voltage instruction generator circuit 16 with the voltage signal of the voltage detector circuit 17 to generate a pulse-width modulation instruction signal, numeral 19 denotes a base drive circuit which, responsive to the pulse-width modulation instruction signal, generates a gate signal to control the transistor which constitutes the inverter 4.
In the thus constructed apparatus for controlling the speed of an elevator, a three-phase AC voltage is rectified and smoothed through the converter 2 and the capacitor 3. An AC voltage of which the pulse width is modulated by the inverter 4 is then supplied to the electric motor 5, and the cage 9 starts to run.
At this time, the speed detector 6 detects the revolving speed of the electric motor 5, and applies the speed signal 6a of cage 9 to the speed control operation circuit 12 which compares the speed signal 6a with the speed instruction signal 11a to produce an output instruction signal 12a and a slip frequency instruction signal 12b which are applied to the voltage control operation circuit 14 and to the voltage instruction generator circuit 16, respectively.
With the output instruction signal 12a and a setpoint value and with the speed signal of the voltage detector 13 as a value that is fed back, the voltage control operation circuit 14 produces a voltage instruction signal, so that the deviation between these two values will become zero. The phase circuit 15 then receives the voltage instruction signal and controls the thyristors of the inverter 2.
Relying upon the slip frequency instruction signal 12b and the speed signal 6a, the voltage instruction generator circuit 16 produces a voltage instruction signal of a sinusoidal wave that will be applied to the pulse-width modulation comparator 18. With the voltage instruction signal as a setpoint value and the voltage signal of the voltage detector circuit 17 as a value that is fed back, the pulse-width modulation comparator 18 produces a pulse-width modulation instruction signal, so that the deviation between these value will become zero, and applies the pulse-width modulation instruction signal to the base drive circuit 19 which then controls the base current of the inverter 4.
Thus, the voltage and frequency applied to the electric motor 5 are controlled, and the speed of the cage 9 is controlled precisely in accordance with the speed instruction signal 11a of the speed instruction generator 11.
FIG. 2 is a circuit diagram which shows in detail the aforementioned voltage control operation circuit 14 which consists of resistors 20 and 21 of which the ends on one side are connected to the speed control operation circuit 12 and the voltage detector 13, respectively; an operational amplifier 24 of which the inverting input terminal (-) is connected to the ends on the other side of these resistors and of which the output terminal is connected to the phase circuit 15; a resistor 22 and a capacitor 23 are connected between the inverting input terminal (-) of the operational amplifier 24 and the output terminal of the operational amplifier 24; and a comparator 25 of which one input terminal is connected to the output terminal of the operational amplifier 24 and the other input terminal is connected to a reference voltage V.sub.ref, and of which the output terminal is connected to the phase circuit 15.
In FIG. 2, as the voltage instruction signal 12a is applied through the resistor and the voltage signal 13a through the resistor 21, a voltage signal 24a corresponding to the difference between the two input signals is applied to the phase circuit 15. The comparator 25 determines the level of the voltage signal 24a. When the level exceeds a predetermined value, a signal is applied to the phase circuit 15 to regenerate the electric power back to the power source, i.e., a bank switching signal 25a of the converter 2 is applied to the phase circuit 15.
In the speed control apparatus of this type, if the power source voltage is denoted by V.sub.ac, and a maximum output voltage of the converter 2 by E.sub.D while neglecting the voltage drop of the thyristor, there exists a relation E.sub.D =1.35 V.sub.ac [V].
If now the power source voltage V.sub.ac drops due to some reasons, the output voltage of the converter 2 drops, also. Therefore, the output voltage of the operational amplifier 24 increases to compensate for the voltage drop. When the voltage drop is relatively great, however, the output voltage signal 24a of the operational amplifier 24 saturates from a time t.sub.1 to a time t.sub.2 as shown in FIG. 3, and the voltage instruction signal 12a often becomes greater than the voltage signal 13a. Namely, the output corresponding to the voltage instruction signal 12a is not always obtained. In this case, if the inverter 4 is shifted from the powering operation to the regenerative operation, operation of the comparator 25 is delayed due to the saturation of the operational amplifier 24, and the voltage rises across the terminals of the capacitor 3. Further, since the power source voltage is low, it becomes difficult to control the current on the regenerative side; i.e., an excess current flows into the thyristor bank on the regenerative side.